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P0140: 1/2 O2 Sensor Stays At Center

NOTE: For connector terminal ID, see CONNECTOR IDENTIFICATION  . For wiring diagram, see WIRING DIAGRAMS article.
NOTE: The 1/2 O2 sensor is referred to as downstream oxygen sensor in this test. Downstream oxygen sensor operation is monitored when engine is running for more than 2 minutes and engine coolant temperature is more than 170°F (76.7°C) with engine in closed loop. If downstream oxygen sensor signal voltage is .29-.72 volt for 30 seconds, this causes Powertrain Control Module (PCM) to pull O2 sensor voltage to 5 volts for 60 seconds and monitors downstream oxygen sensor voltage. DTC may be stored in PCM if downstream oxygen sensor voltage remains at 5 volts. Possible causes are; defective downstream oxygen sensor, defective PCM, or defective connectors or wiring.
  1. Turn ignition on with engine off. Using scan tool, monitor downstream oxygen sensor voltage for a few minutes until downstream voltage stabilizes at .4-.6 volt. Downstream oxygen sensor voltage may also be referred to as 1/2 O2 sensor voltage. Once downstream oxygen sensor voltage is .4-.6 volt, use scan tool to actuate oxygen heater test while monitoring downstream oxygen sensor voltage for 2 minutes. Oxygen sensor heater test may also be referred to as O2 sensor heater test. If downstream oxygen sensor voltage remains .4-.6 volt, go to next step. If downstream oxygen sensor voltage does not remain .4-.6 volt, go to step  9 .
  2. Turn ignition off. Disconnect downstream oxygen sensor connector. Downstream oxygen sensor is located on exhaust pipe, behind catalytic converter.
  3. Inspect downstream oxygen sensor connector for corroded, pushed out, miswired or damaged terminals. If terminals are not corroded, pushed out, miswired or damaged, go to next step. If terminals are corroded, pushed out, miswired or damaged, repair as necessary. Perform TEST VER-5A  .
  4. Using ohmmeter, check resistance between ground and terminal No. 3 (Black/Light Blue wire) on wiring harness side of downstream oxygen sensor connector. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open on Black/Light Blue wire between downstream oxygen sensor and PCM. PCM is located at driver's side front corner of engine compartment, just below air cleaner assembly. Perform TEST VER-5A  .
  5. Connect jumper wire between terminals No. 1 (Tan/White wire) and No. 3 (Black/Light Blue wire) on wiring harness side of downstream oxygen sensor connector. Turn ignition on with engine off. Using scan tool, read downstream oxygen sensor voltage. If downstream oxygen sensor voltage is .2 volt or more, remove jumper wire and go to next step. If downstream oxygen sensor voltage is less than .2 volt, replace downstream oxygen sensor. Perform TEST VER-5A  .
  6. Turn ignition off. Disconnect PCM connectors. PCM is located at driver's side front corner of engine compartment, just below air cleaner assembly.
  7. Using ohmmeter, check resistance between terminal No. 1 (Tan/White wire) on wiring harness side of downstream oxygen sensor connector and terminal No. 51 (Tan/White wire) on PCM connector C2. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open on Tan/White wire between PCM and connector for downstream oxygen sensor. Perform TEST VER-5A  .
  8. Inspect PCM connectors for corroded, pushed out, miswired or damaged terminals. If terminals are not corroded, pushed out, miswired or damaged, replace PCM. Perform TEST VER-5A  . If terminals are corroded, pushed out, miswired or damaged, repair connector as necessary. Perform TEST VER-5A  .
  9. Turn ignition off. Disconnect downstream oxygen sensor connector. Downstream oxygen sensor is located on exhaust pipe, behind catalytic converter. Inspect downstream oxygen sensor connector for corroded, pushed out, miswired or damaged terminals. If terminals are not corroded, pushed out, miswired or damaged, go to next step. If terminals are corroded, pushed out, miswired or damaged, repair connector as necessary. Perform TEST VER-5A  .
  10. Ensure ignition is off. Disconnect PCM connectors. PCM is located at driver's side front corner of engine compartment, just below air cleaner assembly. Inspect PCM connectors for corroded, pushed out, miswired or damaged terminals. If terminals are not corroded, pushed out, miswired or damaged, test is complete. Perform TEST VER-5A  . If terminals are corroded, pushed out, miswired or damaged, repair connector as necessary. Perform TEST VER-5A  .
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